#include <stdio.h>
int main()
{
	char grade='F';
	double t1,t2,a1,a2,total;

	scanf("%lf%lf%lf%lf",&t1,&t2,&a1,&a2);
	total = (t1+t2+a1+a2)/4.0;

	if(total>=80.0) grade = 'A';
	if(total>=60.0 && total<=79.9) grade = 'B';
	if(total>=50.0 && total<=59.9) grade = 'C';
	if(a1<50.0 || a2<50.0) grade--;
	if(grade=='D') grade = 'F';
	if(t1<50.0 || t2<50.0) grade = 'F';
	printf("%c %.1lf\n",grade, total);
}